1. Field of the Invention
The present invention relates an image forming apparatus such as an electrophotographic apparatus, an electrographic recording apparatus or the like having an electrification device for charging an image bearing member such as a photosensitive member or a dielectric member.
2. Related Background Art
An image forming apparatus such as an electrophotographic apparatus conventionally requires an electrification step of uniformly electrifying an image bearing member for forming an electrostatic latent image on the image bearing member and used as electrifying means is a corona electrification device or the like which is not brought into contact with the image bearing member. However, the corona electrification device poses problems in that it produces a large amount of ozone, and that is requires applying a high voltage on the order of 10 kV between the electrification device and the image bearing member.
As an electrification device which solves these problems, there has recently been proposed and put to practical use the so-called contact electrification device which electrifies an image bearing member uniformly by applying a voltage to an electrifying member which is in direct contact with the image bearing member.
A) Roller Charging Device
A representative of the above described contact electrification device is a roller electrification device 2-X shown in FIG. 9.
An electrification member 2-X-a of the roller electrification device is a roller (electrification roller) having a layer of medium resistance disposed on an electrically conductive base roller which is kept in contact with an image bearing member 1 under a predetermined pressure, rotatably held by a bearing and rotated in a direction indicated by an arrow b while following a rotation of the image bearing member 1 which is rotatingly driven in a direction indicated by an arrow a. A predetermined bias voltage is applied from a power supply S1 between the above described roller 2-X-a and the image bearing member 1, whereby the above described image bearing member 1 is uniformly electrified at a uniform potential.
Applied as the above described voltage is (1) only a DC voltage or (2) a DC voltage overlapped with an AC voltage. In case of (1), a voltage on the order of xe2x88x921300 V must be applied to obtain a potential of xe2x88x92600V on the image bearing member 1 or in case of (2), a potential on the image bearing member 1 can be set similarly at xe2x88x92600 V by applying a DC voltage of xe2x88x92600 V overlapped with an AC voltage not lower than 1500 Vpp.
In either case of (1) or (2), an electrification mechanism is governed by Paschen""s law and the image bearing member 1 is electrified owing to a discharge phenomenon which takes place in an area such as an area H shown in FIG. 9 where Paschen""s law is satisfied at a specific distance between the above described electrification roller 2-X-a and the image bearing member 1.
As understood from the electrification mechanism described above, however, such a contact electrification device causes discharge which is the same as that caused by the initially described corona electrification device in the minute spatial area H and also produces ozone though in an amount remarkably smaller than that produced by the corona electrification device. This ozone produces nitric oxide which has low resistance and a defective image is formed due to improper electrification when the nitric acid adheres to the image bearing member 1.
B) Injection Electrification Device
Japanese Patent Application Laid-Open No. 6-3921 or the like proposes an injection electrification process which is free from a problem of ozone production and therefore permits lowering a voltage to be applied to an electrification device.
This electrification process is characterized by being capable of setting a surface potential of an image bearing member nearly at a voltage applied to an electrification device without using a discharge phenomenon but by injecting electric charges to the image bearing member through direct electric charge transfer from the electrification device to a surface of the image bearing member which is brought into contact with the electrification device.
(1) Magnetic Brush Electrification Device
Several types of injection electrification devices are proposed as electrification device for realizing the above described injection electrification process.
As a representative one of the injection electrification devices, a magnetic brush type electrification device 2-Y is shown in FIG. 10. This electrification device is configured by a magnet 2-Y-a, a nonmagnetic electrification sleeve 2-Y-b which comprises the magnet 2-Y-a, a magnet carrier (magnetic carrier, magnetic powder) 2-Y-c, electrically conductive regulating blade 2-Y-d, a housing 2-Y-e and the like. The magnet carrier 2-Y-c is a substance which is magnetic and electrically conductive.
The electrification sleeve 2-Y-b is rotatably disposed in the housing 2-Y-e and a portion of a circumferential surface of the electrification sleeve 2-Y-b is exposed outside through an opening of the housing. The electrification device 2-Y is disposed in a condition where the portion of the electrification sleeve 2-Y-b exposed outside is opposed to the image bearing member 1 with a predetermined slight gap reserved. The magnet 2-Y-a is fixed so as not to rotate. The magnet carrier 2-Y-c is reserved in the housing 2-Y-e. The regulating blade 2-Y-d is disposed in the opening of the housing 2-Y-e so as to reserve a predetermined gap from the electrification sleeve 2-Y-b. 
The magnet carrier 2-Y-c in the housing 2-Y-e is magnetically adsorbed and held as a magnetic brush to the circumferential surface of the electrification sleeve 2-Y-b by a magnetic field of the magnet 2-Y-a, conveyed as the electrification sleeve 2-Y-b rotates, regulated to a predetermined layer thickness by the regulating blade 2-Y-d, conveyed outside through the opening of the housing 2-Y-e, brought into contact with a surface of the image bearing member 1, frictionally slides on the surface of the image bearing member, and is returned and conveyed into the housing 2-Y-e as the electrification sleeve 2-Y-b rotates successively.
The image bearing member 1 is rotatingly driven in a direction indicated by an arrow a and the electrification sleeve 2-Y-b is rotatingly driven in a direction indicated by an arrow s reverse to a rotating direction of the image bearing member 1 in a contact portion (electrification portion) between the image bearing member 1 and the magnetic brush of the magnet carrier 2-y-c. In other words, the magnetic brush of the magnet carrier 2-Y-c rotates at a peripheral speed different from that of the image bearing member 1 as the electrification sleeve 2-Y-b rotates, thereby frictionally slides on the surface of the image bearing member 1.
In the magnetic brush electrification device 2-Y taken as this example, a DC voltage, for example, of xe2x88x92600 V is applied as an electrification bias voltage from a power supply S1 to the regulating blade 2-Y-d. Accordingly, a portion of the image bearing member 1 which is in contact with the magnetic brush of the magnet carrier 2-Y-c tends to be at the same potential. The image bearing member 1 is electrified at this time when electric charges are injected from the magnet carrier 2-Y-c to the image bearing member 1 beyond an energy barrier or electrification does not take place when electric charges move again from the image bearing member 1 to the magnet carrier 2-Y-c at a step the magnet carrier 2-Y-c separates from the image bearing member 1. This phenomenon is largely dependent on a surface energy barrier and electric charge holding capability of the image bearing member 1, whereas a frequency of occasions of contact between the magnet carrier 2-Y-c and the image bearing member 1 is important when the phenomenon is considered as a competitive reaction.
In order to enhance this contact frequency, it is effective to enhance a density of the magnetic brush by reducing a particle diameter of the magnet carrier 2-Y-c and strengthening a magnetic force of the magnet 2-Y-a, and enhance a rotating speed of the magnetic brush of the magnet carrier 2-Y-c relative to that of the image bearing member by setting the rotating direction s of the electrification sleeve 2-Y-b reverse to the advancing direction a of the image bearing member 1 in the electrification portion, thereby bringing the magnet carrier 2-Y-c into contact with the image bearing member 1 more times per unit time.
When the magnet carrier 2-Y-c which is a site to inject electric charges to the image bearing member 1 is brought into contact with the image bearing member 1 with a high frequency as described above, the surface potential of the image bearing member 1 is set at a potential nearly equal to xe2x88x92600 V which is applied to the regulating blade 2-Y-d, thereby permitting uniform electrification free from ununiform electrification even in a microscopic area.
(2) Fur Brush Electrification Device
Furthermore, an injection electrification device 2-Z which uses a fur brush roller 2-Z-a as an electrification member as shown in FIG. 11 is contrived an injection electrification device of a type different from the above described magnetic brush type.
In the fur brush type injection electrification device, fur has a role of the magnetic brush of the magnet carrier 2-Y-c in the above described magnetic brush electrification device 2-Y.
The fur brush roller 2-Z-a has electrically conductive soft fur planted at a high density so that a tip of the fur is in contact with the surface of the image bearing member. The image bearing member is rotatingly driven in a direction indicated by an arrow a, whereas the fur brush roller 2-Z-a is rotatingly driven in a portion (electrification portion) in contact with the image bearing member 1 in a direction indicated by an arrow s reverse to a rotating direction of the image bearing body 1. In other words, the fur brush roller 2-Z-a rotates at a peripheral speed different from the of the image bearing member 1 and frictionally slides on the surface of the image bearing member 1.
A predetermined DC voltage is applied from a power supply S1 to the fur brush roller 2-Z-a as the electrification bias voltage, whereby the surface of the image bearing member 1 is electrified.
(3) Sponge Roller Electrification Device
Furthermore, an injection electrification device 2-A of a type which uses an electrification sponge roller 2-A-a as an electrification member as shown in FIG. 12 is contrived as a type different from the above described magnetic brush type or fur brush type.
In the injection electrification device 2-A, electrically conductive particles Z having relatively low resistance which is referred to as electrification accelerating particles adhere to porous portions (hole portions) of a surface of the electrification sponge roller 2-A-a which rotates in contact with the image bearing member 1, and the electrification accelerating particles Z correspond to the magnet carrier 2-Y-c of the above described magnetic brush type and constitute an injection site.
The image bearing member 1 is rotatingly driven in a direction indicated by an arrow a, whereas the electrification sponge roller 2-A-a is rotatingly driven in a portion (electrification portion) in contact with the image bearing member 1 in a direction indicated by an arrow s reverse to a rotating direction of the image bearing member 1. In other words, the electrification sponge roller 2-A-a rotates at a peripheral speed different from that of the image bearing member 1 and frictionally slides on the surface of the image bearing member 1. In this case, the electrification accelerating particles Z are interposed between the electrification sponge roller 2-A-a and the image bearing member 1. A predetermined DC current is applied to the electrification sponge roller 2-A-a as an electrification bias voltage from a power supply S1, whereby the surface of the image bearing member 1 is electrified.
In any case of the magnetic brush electrification device 2-Y (FIG. 9), the fur brush electrification device 2-Z (FIG. 10) and the sponge roller electrification device 2-A (FIG. 11), however, a number of contacts between the injection site and the image bearing member 1 is important for the above described injection electrification device to maintain an electrification capability, and it is necessary for efficiently increasing the number of contacts to enhance a relative speed by rotating the electrification member in the direction c reverse to the rotating direction a of the image bearing member and enhance a contact pressure of the electrification member so as to increase a contact area between the image bearing member 1 and the electrification member. Accordingly, friction between the image bearing member 1 and the electrification member is enhanced, thereby posing a problem of service lives of both the member which are shortened by deterioration caused due to abrasion.
In the sponge roller type injection electrification device 2-A in particular, a contact pressure is high between the image bearing member 1 and the sponge roller 2-A-a used as the electrification member, the electrification accelerating particles Z having relatively high hardness are interposed between these members and functions like an abrasive, thereby posing a problem that the image bearing member 1 is badly damaged and a surface layer of the image bearing member 1 is cut off rather remarkably.
Means effective for reducing a cut amount of the image bearing member 1 in the image forming apparatus which uses the roller electrification device 2-X (FIG. 9) utilizing discharge are exemplified as follows:
(a) lowering a pressure of a cleaning member which is in contact with the image bearing member 1
(b) lowering an AC voltage to a minimum required level in a case where a DC voltage overlapped with an AC voltage is to be applied to the electrification roller 2-X-a 
(c) shortening a time to apply a voltage between the image bearing member 1 and the electrification roller 2-X-a 
However, it is known that the injection electrification devices described here do not always have means (a) or (b), scarcely damage the image bearing member 1 in (c) by applying voltages and cut the surface of the image bearing member 1 only by a physical force generated simply by friction.
Accordingly, the sponge roller 2-A-a used as the electrification member always slides on the image bearing member 1 while the image bearing member 1 is rotating in the image forming apparatus which uses the sponge roller type injection electrification device 2-A. In other words, this image forming apparatus allows the image bearing member 1 to be cut even at a pre-rotation process, a postrotation process, a sheet interval process and the like where the conventional example is capable of preventing the cutting of the electrification bias, thereby posing the problem that the service life of the image bearing member 1 is shortened.
Though it is conceivable to thicken the image bearing member 1 for prolonging the service life of the image bearing member 1, a thicker film degrades reproducibility of a latent image and blurs an image, thereby posing a problem that prolonged service life of the image bearing member is hardly compatible with a good image quality.
In case of this example, cutting of the image bearing member 1 is largely different, or three or more times as remarkable in not a few cases between operation modes of the image forming apparatus for successive sheet passage and independent sheet passage at processes including the prerotation process and the postrotation process.
It is therefore necessary to assume a maximum cut amount for determining a required thickness of the image bearing member, but there is posed not only a problem that such a thickness imposes too strict specifications and a waste cost on a user who operates an image forming apparatus frequently in the successive paper passage mode but also a problem that such a thickness obliges an image to be more or less lowered in a quality due to the above described degradation of the latent image.
In case of a cleanerless system which does not use a cleaner for cleaning the image bearing member and must have a electrification capability high enough to prevent the sponge roller 2-A-a from being influenced by a developer adhering in a small amount to the roller, there is posed a problem that the above described sponge roller 2-A-a must have a higher contact pressure and cuts the image bearing member in a larger amount.
An object of the present invention is to provide an image forming apparatus which is capable of reducing abrasion of an image bearing member and electrification means.
Another object of the present invention is to provide an image forming apparatus which uses electrification means for injection electrification and is capable of maintaining a high image quality for a long time.